High-performance fibers are used for a multitude of applications where strength, durability, and often weight become an issue. Some fibers have been engineered to match and even exceed the mechanical properties of metals. The term “high-performance fiber” can have various definitions ranging within thermal, chemical, and mechanical properties. For instance, a high-performance fiber could have a storage modulus well within the hundreds of gigapascals range. Commodity fibers, in comparison, are not engineered to perform on the same level; a shirt is not designed for the same level of flame resistance as a fireman's jacket or rip resistance as a soldier's battle dress uniform (BDU). Commodity fibers are typically used in clothing and apparel, upholstery, and general everyday use items such as umbrellas, suitcases, and bags. Protective materials can be an area of overlap between high-performance materials and commodity materials, as well, but the requirements are more extreme for high-performance materials. Boots, for example, are worn to protect the feet from the elements, but specialized composite toe boots can be produced to prevent impact damage at a greater capacity without the weight of steel plating.
In terms of physical strength, high-performance fibers are known to have high breaking strengths, moduli, and durabilities. DuPont's KEVLAR® brand and NOMEX® brand aramid fibers, Toyobo's ZYLON® brand polyoxazole fibers, and Honeywell's SPECTRA® brand fibers are all well-known high-performance fibers utilized across several fields from law enforcement, aeronautics, and military applications. These materials replace heavier metal wires and glass fibers for similar performance at a reduced weight.
Rigid high-performance fibers are typically complex polymers with aromatic structures within the backbone chains. These structures can also have a series of secondary bonding between chains, lining up the aromatic rings to form a “plate-stacking” effect, increasing strength and rigidity. Para-aramids, benzobisazoles, and carbon fiber are among the most noted and high-performance rigid rod polymers. While para-aramids and benzobisazoles have their advantages, their strengths lie in flexural capabilities and energy absorption and redistribution. These fibers are engineered for initial strength, but often miss the mark in terms of longevity and sustainability. In industry, carbon fiber is often preferred for structural composites due in part to the higher axial stiffness and tenacity with drastically reduced weight. Carbon fiber is also resistant to moisture and direct sunlight, unlike para-aramids or benzobisazoles which will degrade without proper shielding.
Like other rigid high-performance polymers, carbon fiber structures are strong due to high alignment and additional secondary bonding. The typical “ladder” structure is highly ordered and can form strong secondary bonds relatively easily. The process to fabricate carbon fiber essentially heats a precursor fiber above 1000° C. until cyclization occurs and reorganizes the backbone chain into a series of rings. Further graphitization can be utilized to remove any inorganic compounds, but requires extreme heat that can reach temperatures in excess of 2500° C. This carbonized structure of precursor fibers, rather than polymerization and fiber spinning, sets carbon fiber apart from typical rigid fibers.
Carbon fiber can be derived from a variety of sources. Pitch and cellulose are representative sources. “Pitch” is a term used for polymeric aromatic hydrocarbons, often comprised of naphthalene, and cellulosic fibers used for carbon fiber production are typically synthetic or recovered cellulose. Ease of processing, defects, and carbon yield are all considered when industry decides which precursor to utilize. Typically, high carbon yield means high efficiency in producing carbon fiber with minimal loss of mass, reducing costs (see Kadla et al. (2002) Lignin-based carbon fibers for composite fiber applications. 40 Carbon 2913). Precursors are desirably as homogenous as possible in order to produce high volumes of high-performance carbon fibers. The commonality among the precursors is the potential for a regular aromatic ladder structure, however this structural regularity and order is necessary for high-performance fiber formation since defects or inconsistencies can act as propagation points for crazing and microcracking that lead to poor quality products (see e.g., Adanur (1995) Wellington Sears Handbook of Industrial Textiles, Technomic Publishing Company, Inc., Lancaster, Pa.).
Due to the unorganized and short length features of natural cellulose fibers, regenerated cellulose is required for producing cellulose-based carbon fiber (Karacan & Gül (2014) Carbonization behavior of oxidized viscose rayon fibers in the presence of boric acid-phosphoric acid impregnation. 49 Journal of Material Science 7462). Regenerated cellulose, also known as “rayon,” is a bio-based alternative source for carbon fiber. Typically, carbon fiber rayon is derived from cellulose recovered from wood pulp. The cellulose is dissolved and then extruded through a spinneret in order to form the rayon filaments. This is an important step that realigns the crystals that can be carbonized and graphiticized (Karacan & Gül (2014)). Carbonization behavior of oxidized viscose rayon fibers in the presence of boric acid-phosphoric acid impregnation. 49 Journal of Material Science 7462; Dumanli & Windle (2012) Carbon fibres from cellulosic precursors: a review. 47 Journal of Material Science 4236). Rayon also requires chemical pre-processing in order to be carbonized, adding labor and chemical costs to the manufacture of carbon fiber. Unfortunately, carbon disulfide is often used for the production of rayon and is highly toxic. Theoretically, rayon could produce high strength carbon fiber, but its processing would require far greater processing temperatures in order to compete with pitch-based carbon fiber (Matsumoto (1985): Mesophase pitch and its carbon fibers. 57 Pure and Applied Chemistry 1553; Zhang et al. (2014) Effect of hot stretching graphitization on the structure and mechanical properties of rayon-based carbon fibers. 49 Journal of Material Science 673).
Derived from plant matter, lignocellulosics are a renewable source for aromatic compounds and easily the most abundant bio-based materials on Earth. They comprise semi-crystalline cellulose and non-crystalline hemicellulose and lignin. Lignin is a prime candidate for carbon fiber production due to the aromatic macromolecular structure, worldwide abundance, and label as a waste product by the paper pulping industry. Paper mills often burned lignin runoff for energy, however the lignin is inefficient as an energy source when compared to petroleum-based fuels, leaving this use less prevalent as industry grows to produce more paper at lower costs (Norberg (2012) Carbon fibres from Kraft lignin, KTH Royal Institute of Technology, Stockholm, Sweden; Brodin et al. (2008) Kraft lignin as feedstock for chemical products: the effects of membrane filtration. 63 Holzforschung 290). Lignin is an appealing source for carbon fiber due to its availability as a by-product or waste from the steadily growing paper pulping industry, however the performance of lignin-based carbon fiber is often lacking in terms of strength and is more comparable to isotropic pitch carbon fibers (Kadla et al. (2002) Lignin-based carbon fibers for composite fiber applications. 40 Carbon 2913).
Despite on-going efforts in the art, there remains a need for methods for preparing lignin-containing fibers, such as lignin-containing poly(vinyl alcohol) (PVA) and/or polyacrylonitrile (PAN) fibers. The presently disclosed subject matter addresses this and other needs in the art.